1. Field of the Invention
The present invention relates to a thermal printing method and a thermal printer. More particularly, the present invention relates to a thermal printing method and a thermal printer with high fidelity in gradation reproduction of an image.
2. Description Related to the Prior Art
There are plural types of thermal printers, including a thermal transfer type and a direct thermal recording type. The thermal transfer type is either a wax-transfer type or a thermal sublimation type. Ink film is superposed on a recording material or a sheet. A thermal head presses the ink film against the recording material and applies heat to it, so that the ink of the ink film is transferred to the recording material. In the direct thermal recording type, the recording material with a thermosensitive characteristic is heated by the thermal head to develop color of the recording material with an image. The thermal head is constituted by an array of heating elements, which respectively consist of a resistor and are arranged in line. The heating elements are driven by a driver.
In the thermal recording with the thermal head including the heating elements being numerous, a peak voltage of a power source increases considerably when all the heating elements are driven simultaneously. Such an increase in the voltage causes an instantaneous drop in the voltage, and thus irregularity in a reproduced image.
There is a suggestion of decreasing the number of the heating elements being driven simultaneously. The heating elements are grouped into plural groups. Starts of applying heat from the heating elements are set different between the groups. JP-A 3-39262 discloses an idea in which the time of applying heat to the heating elements is divided to plural sections, and the sections are changed over between the respective groups for application of heat energy. This is for the purpose of printing an image with a small capacity of the power source. The heating elements are alternately or cyclically driven between the groups. But the width of drive pulses is divided to plural smaller units. If the printing operation is effected at a high speed, the width of the drive pulses is decreased. Differences in the time of turning on and off the heating elements will remain in results of the printing in a form of irregularity in gradation reproducibility, which will be degraded remarkably.